Imagine that you have an older web scenario (not related to this issue itself) where {sid} macro stopped to be expanded.

In this case zabbix frontend receives this GET request:

index.php?reconnect=1&sid={sid}

which causes this error to be logged to Apache/PHP error log:

[Tue Sep 01 10:04:32.059376 2020] [php7:notice] [pid 19750] [client ::1:59660] PHP Notice:  Undefined index: scripts in /zab/www-dev/git-clone/5.0/ui/include/page_header.php on line 170

which is not very clear what is source of this issue.

If try in frontend, we can see that error too:

I think that "Undefined index" error error should not happen in any case, so consider please to fix it.